What is the basic difference between epithelial tissue and connective tissue? Why are they so different despite the fact that all tissues originate in embryo and an embryo arises from a single fertilised egg cell?
1 Answer
Epithelial tissue has almost no matrix while connective tissue has a huge amount of matrix between cells.
Tissue differentiation is achieved by differential gene expression.
Explanation:
Epithelial tissues either protect/cover/line an organ. In modified condition they may form secretory glands, as seen in the wall of stomach and small intestine. Epithelial tissues are formed by a specific type of cells which are arranged in single or multiple layers. The tissue sits on a thin non-cellular basement membrane which does not allow blood vessels to directly penetrate epithelium. Exposed side of epithelium could be ciliated, as seen in the lining of trachea, oviduct, etc.
Connective tissues provide support as skeletal tissue, as packaging material, and most importantly as blood. Matrix of a connective tissue could be solid, as in bones or jelly like, as in adipose/areolar or liquid, as in blood. Matrix may possess fibres: collagen, elastin, etc. Extensive blood vessels supply oxygen in connective tissue. In a single connective tissue two or more types of cells could be present,for example, RBCs, WBCs and Platelets in blood. .
All the differences that we see between tissues are genetic in origin. It is important to note that all connective tissues develop from embryonic mesoderm but epithelial tissues may arise from all germ layers. Thus, epithelial layer of skin is ectodermal, innermost mucosal lining of gut is endodermal, while peritoneal lining of coelom is mesodermal in origin.
Different cells achieve different structural and functional characteristics: by keeping many of the genes in switched off condition, while only particular genes express through protein synthesis. Thus keratin protein is present only in skin epithelium, no where else in body. HCl is secreted by only chief cells of gastric epithelium. Likewise, only blood lymphocytes produce antibodies, only fibroblasts secrete fibres in tissue matrix, only chondroblasts can secrete components of cartilage, and the list goes on.
Tissue differentiation takes place very early in embryonic life.
